1. Field of the Invention
The present invention relates generally to magnetic resonance imaging (MRI) guided thermal treatment systems, and more particularly to using MRI to obtain thermal evolution images of a tissue mass undergoing thermal treatment.
2. Background of the Invention
Certain types of body tissues, such as tumors, can be destroyed by heat. One way to apply thermal energy to internal body tissue is to focus high intensity, ultrasonic acoustic waves into the tissue using, e.g., a phased-array of piezoelectric transducers. Such treatment can reduce or even eliminate the need for invasive surgery to remove the tissue. Of critical importance to the treatment process is verifying that a sufficient thermal dose is reached during each application of ultrasonic energy (or xe2x80x9csonicationxe2x80x9d) to kill/ablate the portion of the target tissue structure being heated. It is also important to be able to precisely track which portions of the tissue structure have been killed/ablated, in order to minimize the total number of sonications needed in a particular treatment session.
Towards this end, MRI systems are used to assist in aiming ultrasonic wave energy at a target tissue structure in a body, and to monitor the temperature change of the tissue region being heated during the thermal treatment process to ensure that a sufficient thermal dose is reached to fully ablate the tissue being heated.
One method of measuring temperature change using MRI techniques exploits the temperature dependence of the proton resonant frequency (PRF) in water. The temperature dependence of the PRF is primarily due to temperature-induced rupture, stretching, or bending of the hydrogen bonds in water. The temperature dependence of pure water is 0.0107 ppm/xc2x0 C., and the temperature dependence of other water-based tissues is close to this value. Because of a non-homogenous magnetic field within the MRI machine, absolute PRF measurements are not possible. Instead, changes in PRF are measured by first taking a MR image before the delivery of heat, and subtracting this baseline value from subsequent measurements. The temperature-induced changes in PRF are then estimated by measuring changes in phase of the MR signal, or frequency shift, in certain MR imaging sequences.
Notably, the duration of each sonication must be limited, e.g., to approximately ten seconds, in order to avoid the unwanted (and painful) build up of heat in healthy tissue surrounding the target tissue structure being heated. Thus, there is limited time for acquiring temporal MR images for monitoring the temperature increase during a sonication in order to verify a sufficient kill temperature has been reached. Because it can take at least one and as much as much as three seconds to acquire a single MR thermal sensitive image, this means there is little room for-using a multi slice MR imaging techniques to cover the whole heated volume.
The invention is directed to systems and methods using magnetic resonance imaging for monitoring the temperature of a tissue mass undergoing thermal treatment by energy converging in a generally elongate focal zone symmetrical about a focal axis.
In one embodiment, a system is configured to acquire a first plurality of images of the tissue mass in a first image plane aligned substantially perpendicular to the focal axis. A cross-section of the focal zone in the first image plane is defined from the first plurality of images. A second plurality of images of the tissue mass are also acquired in a second image plane aligned substantially parallel to the focal axis, the second image plane bisecting the first image plane at approximately a midpoint of the defined focal zone cross-section.
In one embodiment, the first plurality of images includes a baseline image taken prior to the application of ultrasound energy to the tissue mass.
In one embodiment, one image of the second plurality of images is acquired proximate the end of an ultrasound sonication.
In one embodiment, the system is configured to further acquire one or more additional images of the tissue mass in the focal zone in a third image plane aligned substantially perpendicular to the focal axis while acquiring the images of the second plurality of images. A cross-section of the focal zone in the third image plane is then defined from the additional images, whereby it may be verified that the second imaging plane bisects the third image plane at approximately a midpoint of the defined focal zone cross-section in the third image plane.
In preferred embodiments, the system is further configured to derive a three-dimensional thermal evolution of the tissue mass in the focal zone based on the first and second pluralities of images. In one embodiment, the thermal evolution is derived by defining a cross-section of the focal zone in the second image plane from the second plurality of images, the cross-section of the focal zone in the second image plane having a length. The cross-section of the focal zone in the first image plane is extrapolated along the length of the cross-section of the focal zone in the second image plane. The thermal evolution is then generated based on differences in a characteristic of the tissue mass in the focal zone measured in successive images of the second plurality of images, wherein the differences correspond at least in part to changes in temperature of the tissue mass between respective images.
In one embodiment, the measured characteristic is a phase of an electromagnetic signal emitted from the tissue mass, and wherein the corresponding temperature change is derived from a phase shift in the signal between successive images.
In one embodiment, the thermal evolution is used to verify that the temperature of the tissue mass in the focal zone exceeded a threshold temperature.
Other aspects and features of the invention will become apparent in view of the disclosed and described embodiments.